Desmid of the month
October 2011

Actinotaenium
phymatosporum

Actinotaenium phymatosporum (in most floras listed as Penium phymatosporum) may be readily confused with Actinotaenium spinospermum (Penium spinospermum). Both species are characterized by small cell dimensions and a faintly striate cell wall (often the striae are hardly visible). Actually, the striae are made of longitudinal series of closely set cell wall pores, better to be distinguished as the pores are marked by protruding excretion products. Owing to the nature of this cell wall sculpturing both above-mentioned species were transferred from Penium to Actinotaenium (Kouwets & Coesel 1984). Vegetative cells in A. phymatosporum are slightly longer than in A. spinospermum but the main difference is in the shape of the zygospore: irregularly rectangular in A. phymatosporum versus about globose in A. spinospermum. In the Netherlands, A. phymatosporum is known from acidic, oligo-mesotrophic, shallow water bodies. Zygospores are but rarely encountered.